Triton (moon)

Triton, Neptune’s largest satellite, is the solar system’s only major satellite in a retrograde orbit. It has smoke plumes from geyser-like eruptions that astronomers and planetologists cannot explain. Triton appears younger than most satellites and planets in the solar system.

Overview

The Neptunian satellite Triton is 2,706 kilometers in diameter. The satellite’s density is 2.059 grams per cubic centimeter. With this density—higher than any other outer planet satellite that has been measured, models of the satellite can be constructed. Scientists posit there is a metallic and rocky core, a silicate mantle, a layer of ice, a possible ocean, and a top layer of ice. The core has a radius of about six hundred kilometers, while the mantle has a thickness of about 350 kilometers, with a 150-kilometer layer of ice below the ocean and a 250-kilometer layer above the ocean.

William Lassell, a brewer by trade, discovered Triton on October 10, 1846, using a telescope he built. Sir John Herschel had asked an amateur astronomer, Lassell, to look for satellites of the newly discovered planet Neptune—found just seventeen days earlier. Triton is approximately 355,000 kilometers from Neptune. The eccentricity of Triton’s orbit is zero, meaning that the orbit is circular. Triton is a most unusual satellite. Its orbit is retrograde—it rotates around Neptune in the opposite direction to the rotation of Neptune. It is synchronous with Neptune, meaning it always presents the same face as Neptune. Being synchronous also means that the rotation time of Triton is the same as the time for one orbit around Neptune, five days and twenty-one hours (or 5.877 Earth days). The angle of orbital inclination is twenty-three degrees if one ignores its retrograde motion; including retrograde motion, the moon's orbital inclination is technically about 157 degrees.

This large angle of inclination, coupled with the retrograde direction of Triton’s orbit, suggests that Neptune's gravitational field captured Triton from the Kuiper Belt. The method by which this capture occurred is unknown. Triton may have collided with another satellite, causing Triton to slow down enough to be captured and destroy the other satellite simultaneously. Another idea is that the other satellite was knocked out of orbit, and Triton was captured. A third idea is that Triton was part of a binary system. Triton was captured; however, the partner escaped. When Triton was captured, the orbit was probably highly elliptical. The gravitational force of Neptune gradually changed Triton’s orbit into the current circular orbit. During this change, the strong pull when Triton was close and the weaker pull as Triton was far away would have caused tidal flexing or movement within Triton’s structure. Such motion would have caused internal friction, generating heat. The heat would have caused differentiation, that is, separation of the components of Triton. Heavier materials, such as metals, would have sunk to the core; medium-mass materials, such as silicates, would have formed a mantle; and lighter materials would have been forced to the surface.

Many believe that Triton is a volcanic satellite because of plumes of dark material that appear to be blown from its surface into the air. This material is concentrated enough to be easily seen. The plumes are about two kilometers across, rise as high as eight kilometers, and consist of particles less than two millimeters in diameter. The particles’ sizes can be inferred because they do not settle to the surface. These plumes may be putting out ten kilograms of material per second and may last for years.

Triton has a thin atmosphere composed predominantly of nitrogen at fourteen millibars of pressure (Earth’s atmospheric pressure is about one bar). There are clouds composed of condensed nitrogen. A diffuse haze is also observed. The haze likely consists of hydrocarbons and nitriles produced by the action of sunlight on methane. Wind-driven streaks are oriented in an east-west direction. The wind causes some of the streaks by material blown from the plumes. When all the streaks, clouds, and plumes are considered, the winds blow northeast close to the surface, eastward at intermediate levels, and westward at the top of the troposphere.

Infrared technology gave scientists the first look at Triton’s surface. The spectra could be modeled only by a combination of solid methane (CH₄), called methane ice, liquid nitrogen, and water ice. Later spectra showed nitrogen (N₂) in a solid or liquid form. One idea is that there is a sea of nitrogen with small amounts of dissolved methane. More likely, there is a layer of solid nitrogen with contaminants of methane, carbon dioxide (CO₂), and carbon monoxide (CO). Even at the measured atmospheric temperature of thirty-eight kelvins (-235 degrees Celsius), the nitrogen ice will sublime, forming the thin atmosphere first found by Voyager 2. The nitrogen refreezes at the winter pole of Triton, causing a polar ice cap. Solid nitrogen is very transparent; therefore, the sunlight that does reach Triton can heat the interior of the solid nitrogen in a greenhouse effect. Nitrogen, in either gaseous or liquid phase due to heating nitrogen originally in solid form, will flow to the surface, where it freezes. Some of the nitrogen will escape into the atmosphere. The layer of nitrogen thus moves, or at least thins, with the seasons. The seasonal shift has plenty of time because Triton has a 688-year climate cycle due to its unique rotational and orbital motion. Thus, there is a higher albedo (0.7) on the winter end of the satellite, where the layer of nitrogen ice is thick, and a lower albedo (0.55) on the summer end of Triton. The summer end will show methane ice, which has a reddish color. Radiation will eventually turn the methane dark. The methane is not dark, so the ice is quickly refreshed.

Triton displays at least three different types of surfaces: a bright polar area, areas of dark patches surrounded by lighter material, and high, walled plains. The bright polar area seems to be nitrogen ice on top of a cantaloupe-like, dimpled surface. The dimples, called cavi, are caused not by volcanoes but by diapirism. Diapirism is generated by gravitational instability in which less dense material flows through denser material. The density gradient may be caused by temperature or a composition difference. The implication of Triton is that the crust has distinct layers and that the top layer is no more than twenty kilometers thick. The area also has linear ridges across the cantaloupe terrain.

Dark patches within the lighter material are called maculae and probably are composed of carbonaceous material, such as methane ice. The bright material is probably nitrogen ice. Maculae may mark spots of heat that have lost the nitrogen ice layer, allowing the methane ice, with its lower albedo, to show through.

A flow of volcanic ice causes the high plains. Some of the tables are smooth plains with a flat-to-undulating structure. Other plains are surrounded by a terraced wall or steppes, called scarps. These plains are very flat, implying that they were filled with liquid at one time. Scarps may be the remainder, as the material on the plains sublimed.

One unique feature of Triton is its small number of craters. There is one crater that is twenty-seven kilometers across, named Mazomba. Still, the small number of craters suggests either that the surface of Triton is very young or that the surface was refreshed fairly recently—or both. Part of Triton’s surface is considered cryovolcanic instead of silicate-magma volcanic. Cryovolcanic activity is the eruption from the subsurface of icy-cold liquids, which then refreeze on the surface in a more or less smooth structure.

Knowledge Gained

Much of what is known about Triton has come from Earth-based instruments and the Hubble Space Telescope (HST). The Hale Observatory used narrow-band spectrophotometry to determine that Triton has a constant spectral reflectance. Astronomers have compared data from HST, the Infrared Telescope Facility at the University of Hawaii, and Voyager 2 for years to see if there is a seasonal change in Triton’s surface. There appears to be a change—possibly every forty years—but since the climate cycle is so long, the data remain inconclusive. Both types of surface composition—methane ice and solid nitrogen—were detected by infrared spectra from an Earth-based instrument.

Voyager 2 provided more information in a short time than the land-based instruments had been able to gather in the years since Triton’s discovery. The spacecraft’s small changes in flight path caused by Triton’s mass allowed that mass to be calculated. Pictures allowed the size to be determined, and density could then be calculated. Models of the structure of the satellite could then be developed. The density, 2.059 grams per cubic centimeter, indicates a large component of silicate materials, even though they do not show in infrared spectra because they are under ice. Voyager also detected the nitrogen atmosphere.

Pictures from Voyager 2 show the effects of wind on Triton, an unexpected phenomenon. Varied terrain and plumes have also been noticed in the photos. The temperature measured—thirty-eight kelvins—is one of the coldest in our solar system. Even with the cold temperature, the different terrains indicate that the surface has been refreshed more recently than any other moon or planet except those geologically active planets or planets. NASA’s James Webb Space Telescope began capturing photos of Neptune's Triton in the early 2020s which were better quality than those obtained by the Voyager missions.

Context

The density of Triton is close to that of the Pluto-Charon system. Is that a coincidental fact, or are Triton and Pluto related objects of the Kuiper Belt? Could Pluto at one time have been a satellite of Neptune that was knocked off by Triton? The orbital inclination, rotational speed, and retrograde motion all point to some cataclysmic occasion that produced Triton as a satellite.

Triton’s surface features raise interesting questions about its energy source. Triton’s is one of the coldest surfaces in the solar system, yet it also appears to be active, given the plumes and smoothness observed. How can these conditions coexist? The idea that enough sunlight penetrates a deep sheet of solid nitrogen to produce a greenhouse effect under the ice is startling but may be true; it does appear that the ice sublimes and then refreezes in another place. Yet where does Triton get the energy to produce plumes rising eight kilometers into the satellite’s tenuous atmosphere? Even given the satellite’s rather low gravitational acceleration, this phenomenon completes the satellite’s overall mystery. Is Triton’s interior heated radiogenically? Is there another heat source?

Scientists may not understand as much about the effect of very low atmospheric pressure, low temperature, and heavy mass as was previously thought because the planet’s heat must be generated somewhere. The theory that Triton is heated radiogenically will have to wait until the subsurface can be monitored for radioactive isotopes or their daughter isotopes before it can be confirmed or disproved. Indeed, the answer to the question of Triton’s energy source will add to our understanding of the solar system's origins and our knowledge of energy physics.

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